An electronic circuit breaker using digital circuitry inserts the primary winding of a respective current transformer into each conductor of a power line it protects; and signal at the secondary winding of each current transformer so employed is rectified and converted to digital form. The resulting samples are squared by means of digital multiplication, and integrated over a time period fifty milliseconds or so long. The integrated squared samples are then accumulated over prescribed periods of time and threshold detected to generate a trip signal, should overcurrent occur over too long an interval of time. A trip signal actuates an electromechanical switch for interrupting the flow of current through each conductor of the power line. Accumulation has been done over a relatively small numbered plurality of samples and the accumulations threshold detected at a relatively high level, to generate a short-time-constant trip signal; and accumulation has been done over a relatively large-numbered plurality of samples and the accumulation threshold detected at a relatively lower level to generate a long-time-constant trip signal as well.
The generation of trip signals as thusfar described is invariably too slow, however, when catastrophic fault conditions are imposed on one or more of the power line conductors. The electromechanical switches used to interrupt the power line conductors can respond to a trip signal in about fifty milliseconds, and it is desired to generate "instantaneous" trip signals in a fraction of that time. One millisecond is the commercial requirement for the analog-to-digital converter and threshold detection apparatus in an electronic circuit breaker to generate instantaneous trip signal. It is desirable that such apparatus be powered directly from the power line conductors the circuit breaker protects, as pointed out by S. E. Noujaim in U.S. Pat. No. 4,768,018 issued Aug. 30, 1989; entitled "ANALOG TO DIGITAL CONVERTER FOR AN ELECTRONIC CIRCUIT BREAKER WITH OUT-OF-SUPPLY-RANGE INPUT SIGNALS" and assigned to General Electric Company. A typical power-up time for such a supply is about 400 microseconds, which leaves only about 600 microseconds thereafter for the analog-to-digital converter and threshold detection apparatus to generate the instantaneous trip signal. So about 1600 conversion results or more have to be generated per second for instantaneous trip to be fast enough to be commercially acceptable. Such rapid conversion rates reduce the amount of time integration of power line current response that can be done in the analog-to-digital converter and threshold detection apparatus. This makes it likely that short-duration, high-current transients on the power line conductors will generate instantaneous trip signals, even when their energy content is insufficiently large to be of concern. That is, "false" trips become a significant problem. An aspect of the invention is reducing the likelihood of false trips by requiring at least two consecutive conversion results indicative of over-current to occur before an instantaneous trip signal is generated. When this is done, about 3200 conversion results or more have to be generated per second for instantaneous trip to be fast enough to be commercially acceptable.